Eight investigators from four departments at Columbia University are applying for funding to purchase an XL-I analytical ultracentrifuge to enhance research capabilities in structural and molecular biology. All of the investigators study interactions of biological molecules. The analytical ultracentrifuge will be used to (i) determine sample homogeneity (degree of polydispersity) in the presence and absence of denaturants, (ii) measure sedimentation and diffusion coefficients, (iii) determine molecular masses, (iv) study oligomerization of macromolecules and measure dissociation constants, stoichiometries and thermodynamic parameters and (v) estimate molecular shape and changes in conformation upon ligand binding. Other applications, currently worked-out and those not yet developed, will be applied as necessary. A wide range of molecules will be investigated including ligand-gated ion channels, self-assembling transmembrane toxins, mesophilic and thermophilic ribonucleases, extracellular matrix proteins (integrin receptors and fibronectin type III domains), cell surface molecules of the immune system and their complexes with HIV proteins, giant invertebrate hemoglobins, Taxol-tubulin complexes, ATP binding cassette transporters and so called two-component signal transduction systems. A detailed structure has been defined for the administration, operation and maintenance of the XL-I facility and the fostering of an educated group of XL-I users. This involves (i) the creation of an XL-I Advisory committee, (ii) a method for allocation of instrument time for major and outside users, (iii) a strategy for continuing education in the field of analytical ultracentrifugation to keep users up-to-date, (iv) a plan for user training, (v) a basis for management of the XL-I facility over the short and long term, and (v)a vision of how we should apply current and as-yet-to-be developed programs for data analysis.